I had been using Spamassassin an a FreeBSD system for a few years when I set out to try and get it working on an Exchange 2000 box. I wrote a hack that allowed that to work, and have documented that before on this site (here and here for example).
For a small amount of mail (certainly say within a thousand messages a day, each, for 20 users) this solution works pretty well, especially once you get it tweaked and have your white/black lists worked out and the Bayesian filters trained.
As noted by this person who tried it, it takes less than a second per email, so the better your hardware, the more you could theoretically handle. Also as that same person notes though, there are better solutions - they ended up switching overto SpamStopsHere (something I will try and cover at a later date since I don't have any personal experience with it, yet).
Do note that the best part of this option is that it is totally free and allows the admin to tinker with it and have as much control as they like. This is admittedly not always a good thing.